1. Field of the Invention
The invention relates generally to the production of cellulosic materials using straw as a raw material and, more particularly, the invention relates to the production of man-made board products using wheat straw as furnish.
2. Description of Related Technology
Man-made board products are generally classified into three categories including softboard (having a density less than about 30 pounds per cubic foot), medium-density fiberboard (having a density between about 30 and about 45 pounds per cubic foot), and hardboard (having a density greater than about 45 pounds per cubic foot).
The principal processes for the manufacture of boards include (a) dry felted/dry pressed or "dry" processes, (b) wet felted/dry pressed or "wet-dry" processes, and (c) wet felted/wet pressed or "wet" processes. In a dry process, cellulosic fibers (e.g., woody material which has been subjected to fiberization to form wood fibers) are generally mixed with resins and conveyed in a gaseous stream or by mechanical means to a support structure to form a mat. The mat is then pressed to create a board product. More specifically, cellulosic fibers supplied from a fiberizing apparatus (e.g., a pressurized refiner) can be first coated with a thermosetting resin binder, such as a phenol-formaldehyde resin, in a blowline blending procedure. The resin-coated fibers from the blowline can then be randomly formed into a mat by air blowing the fibers onto a support member. The fibers, either before or after formation of the mat, can optionally be subjected to pre-press drying, e.g., in a tube-type dryer. The mat, typically having a moisture content of less than 30 weight percent (wt.%), and preferably less than 10 wt.%, is then pressed under heat and pressure to cure the thermosetting resin and to compress the mat into an integral consolidated structure.
In blowline blending, which takes place between the fiberizing apparatus and the pre-press dryer, the binder resin (generally saturated with steam from the fiberizing apparatus) is blended with the fiber using air turbulence. Blowline blending offers several advantages, including ease, quality, and efficiency of blending of the fibers and the binder resin. In many blowline processes, resins are applied to the wet fibers at about 200.degree. F.-250.degree. F. (e.g., approximately 230.degree. F.), and then are passed into a dryer having an inlet temperature of about 320.degree. F. to about 400.degree. F. The inlet temperature depends upon dryer efficiency and/or the diameter of the dryer entry.
A typical wet-dry process begins by blending cellulosic fiber material in a vessel with large amounts of water having an acid or neutral pH (i.e., 7 or less) to form a slurry. This slurry is then blended with a resin binder and the blend is deposited onto a water-pervious support member, where a large percentage (e.g., 50 percent) of the water is removed through vacuum draining and press rolls, thereby leaving a wet mat of cellulosic material having a water content of about 40 wt.% to about 60 wt.%, based on the weight of the wet mat, for example. This wet mat is then transferred to a zone where much of the remaining water is removed by evaporation, preferably facilitated by the application of heat, to form a dried mat. The dried mat preferably has a moisture content of less than about 10 wt.%. The dried mat is then transferred to a press and consolidated under heat and pressure to form the wood composite which may be a flat board or a doorskin article, for example. The product can have any other desired shape, depending on the intended use of the product.
Generally, in a wet process, cellulosic fillers or fibers are blended in a vessel with large amounts of water to form a slurry. The slurry has sufficient water content to suspend a majority of the wood fibers and preferably has a water content of at least 90 wt.% and, most preferably 96 to 98.5 wt.%. The slurry is deposited along with a synthetic resin binder, such as phenol-formaldehyde resin, onto a water-pervious support member, such as a fine screen or a Fourdrinier wire, where much of the water is removed to leave a wet mat of cellulosic material having, for example, a moisture content of about 110 wt.%, based on the weight of dry cellulosic material. The wet mat is then consolidated under pressure and dried in a drying oven to form a wood composite.
Typically, the cellulosic fibers used in each of a dry process, a wet-dry process, and a wet process are produced from wood chips or other low quality forest residues. In forming these fibers, the wood chips or the residues are reduced to a fairly uniform size by chipping, sizing, and/or screening. The wood chips are then cleaned and softened using, for example, a steam thermal process. The softened wood chips are then mechanically refined in a refiner such as an atmospheric or pressurized refiner, which grinds and separates the softened wood chips to form the cellulosic fibers. However, with the depletion of natural wood resources, wood by-products, such as wood chips and other low-grade forest materials, are becoming increasingly more expensive and difficult to obtain. It is desirable, therefore, to use a readily obtainable and abundant material as a replacement for wood chips as furnish in the production of cellulosic materials like man-made board products.
Furthermore, boards made from wood products are usually very flammable. It is desirable, therefore, to replace the wood fibers normally used in board manufacturing processes with fibers that have a greater resistance to fire than the wood fibers.
Wheat straw, which is an abundant and renewable resource, has a number of inherent disadvantages as furnish in the production of cellulosic materials as compared to wood chips and other low-grade forest products. The hollow, tubular shape and waxy outer surface of wheat straw makes the straw less dense than most wood materials and, if not properly preconditioned, causes boards produced therefrom to have less than desirable strength. Furthermore, long strands of straw are undesirable during the board forming process because they tend to wrap around the components of the board manufacturing process which, in turn, causes frequent shut down of the process for cleaning. The cleaning process is labor intensive and interrupts continuous production of boards.
Nonetheless, wheat products, such as wheat straw, have been used as a raw material in some processes that typically rely on the use of wood products as raw materials. It is known, for example, to use straw in a paper-pulping procedure whereby the straw is broken down until completely delignated and formed into a cellulosic component for use in paper-making machines. Hernandez et al., U.S. Pat. No. 4,243,480 (Jan. 6, 1981), discloses a process of producing paper products using water-insensitive starch fibers made from any naturally occurring or fractionated starch, including wheat, to replace all or part of the cellulosic or other pulp conventionally employed in the paper-making process. In this process, the starch fibers are precipitated by the extrusion of a thread-like stream of a colloidal dispersion of starch at 5 wt.% to 40 wt.% solids into a moving, coagulating salt solution. The aqueous suspension of starch fibers produced by the mixture of the starch dispersion and the aqueous salt solution is introduced into the pulp stream of a paper-making process, thereby enabling production of fibers in the paper web. These processes of using wheat products are only applicable to paper-making procedures and, therefore, are considered inadequate for the production of board products.
It is also known to use straw in a dry process to produce particle board products. In such a process, the straw is chopped and/or mechanically ground and bound together with a resin to produce particle board. However, producing particle board having a straw content of greater than approximately 10 wt.% with a dry process usually produces a product which is vastly inferior in strength.
Dvorak, U.S. Pat. No. 4,451,322 (May 29, 1984), discloses a compacting apparatus for forming pressed sheets from crude plant fiber, such as straw from stalks of cereal grains. The fibrous material is first dried to have a moisture content of less than 15 wt.% and is stripped and broken down into small fibrous strains of substantially uniform diameter. The compacting apparatus includes a conveyor which transports the dried, cut fibrous material into a chute which, in turn, delivers the fibrous material to a compactor. The chute includes a plurality of arcuately reciprocating plungers which are serially arrayed along the flow path of the fibrous material and which precompact the fibrous material to have a uniform density as it is delivered to the compactor. In the compactor, a ram periodically operates to compact the fibrous material into a tunnel such that the fibrous material is formed into a continuous sheet having a uniform height and width. After exiting the tunnel, the sheet enters a press where heat and pressure are applied to the surfaces thereof to increase the mechanical interlinking of the individual fibers in the sheet. Surfacing material is bonded to the compacted sheet which can then be cut to any desired length.
These dry processes that use straw do not enable the production of board products using only or a majority of straw as a raw material. These processes may also require specialized and potentially expensive equipment. Furthermore, these processes are not applicable to certain board manufacturing techniques such as wet processes and wet-dry processes. In fact, none of these processes includes a method of refining straw into fibers capable of being used in the production of cellulosic materials in each of a dry process, a wet-dry process, and a wet process.